1. Field of the Invention
The invention relates to a pressure booster for increasing the injection pressure in direct-injection internal combustion engines with a plurality of cylinders, in particular diesel engines, having a booster piston which has a relatively large end face that is subjected to a relatively low pressure and a relatively small end face at which in the pumping stroke an elevated injection pressure prevails, wherein on the side remote from the relatively large end face, because of the difference in size between the two end faces of the booster piston, an idle volume results. The invention also relates to a fuel injection system, in particular a common rail system, with a high-pressure connection that communicates with an injector, from which fuel is injected into the combustion chamber of an internal combustion engine.
2. Description of the Prior Art
For introducing fuel into direct-injection diesel engines, both pressure-controlled injection systems (German Patent Disclosure DE 197 06 467 C1) and stroke-controlled injection systems (DE 196 19 523 A1) are known. It is advantageous if the injection pressure is adapted to the load and rpm, as can be the case in a common rail system.
In common rail injection systems, a high-pressure pump, optionally with the aid of a prefeed pump, pumps the fuel to be injected out of a tank into the central high-pressure fuel reservoir, also known as a common rail. From the rail, fuel lines leads to the individual injectors, which are assigned to the engine cylinders. As a function of the operating parameters of the engine, the injectors are triggered individually by the engine electronics in order to inject fuel into the combustion chamber of the engine. The maximum injection pressures in common rail systems are currently limited, by the high-pressure strength of pressure reservoirs and high-pressure pumps, to about 1600 to 1800 bar. To increase the injection pressure still further, injection systems with pressure boosters are employed.
One such common rail system is described for instance in European Patent Disclosure EP 0 562 046 B1. This involves a pressure-controlled common rail system with a pressure booster (high boosting ratio of approximately 1:7). As in every pressure-boosted injection system, here as well there is an idle volume, in which a restoring spring is disposed. The idle volume communicates with a leakage line via a (springless) check valve. This has the advantage that the quantity positively displaced during the pumping stroke need not be reaspirated from the leakage line upon restoration of the booster piston. A disadvantage, however, is that the fuel located in the idle volume cavitates during the restoration. When the cavitation bubbles pop, they generate additional noise.
Upon activation of the pressure boosting, an increased injection pressure during the pumping stroke is obtained, because of the different ratios of the areas of the faces of the pressure booster piston. Once the injection has ended, the booster piston must return to its outset position (refilling). To that end, usually the low-pressure side of the piston is relieved, and the piston is pressed back into its outset position because of the balance of forces at the piston. During the return motion, the high-pressure chamber of the pressure booster must be filled with fuel. If this is accomplished only with the aid of a prefeed pressure, then at high pressure boosting ratios an additional restoring spring is necessary, since the resultant force at the high-pressure face is not sufficient to overcome the forces of friction at the piston. To achieve restoration without a restoring spring, a high pressure would be needed on the high-pressure side, but this pressure would have to be generated additionally, which would mean a considerable additional expense. In principle, the restoring spring can be mounted in any of the three chambers (idle chamber, high-pressure chamber and low-pressure chamber). Where the spring or springs is disposed depends on the type of piston (in one or two parts) and the installation space available. In low-pressure side relief actions in xe2x80x9cmixed operationxe2x80x9d, restoring springs also increase the restoration speed.
At low pressure boosting ratios, it is problematic to integrate a restoring spring of a suitable stroke-force course, since the installation space is highly limited.
The object of the invention is to disclose provisions which assure adequate, rapid refilling of the pressure booster even at low pressure boosting ratios.
In a pressure booster for increasing the injection pressure in direct-injection internal combustion engines with a plurality of cylinders, in particular diesel engines, having a booster piston which has a relatively large end face that is subjected to a relatively low pressure and a relatively small end face at which in the pumping stroke an elevated injection pressure prevails, wherein on the side remote from the relatively large end face, because of the difference in size between the two end faces of the booster piston, an idle volume results, this object is attained in that the idle volume is in communication with a cantilevered or offset spring reservoir, which is disposed locally for each individual cylinder or centrally for all the cylinders of the engine. By cantilevering or offsetting the spring, a suitable spring geometry (diameter, length, wire thickness, stiffness) can be selected, thus making it possible to optimize the piston shape and hence other favorable effects (friction minimization, sealing lengths, diameter ratios). Another advantage can arise in a central storage chamber/diaphragm chamber that is common to all the cylinders, because in this case the restoration can be the pressure booster. This can be avoided by means of a suitable pressure limiting valve, which diverts the excess quantity. achieved with only a few additional component parts (in restoration purely via a pilot pressure on the high-pressure side, that pressure would have to be generated first, which would impair the efficiency).
A particular embodiment of the invention is characterized in that the spring reservoir includes a piston which is prestressed by a spring. Although the spring does reduce the maximum injection pressure, this favors fast refilling. In the spring chamber, there is enough installation space for a large spring. Optionally, a small spring can also be disposed in the idle chamber, which further reinforces the refilling.
A further particular embodiment of the invention is characterized in that the spring reservoir includes a prestressed diaphragm. The diaphragm can replace the piston.
If no diaphragm is provided, then because of leakage at the spring chamber piston, a communication between the spring restoration chamber with the leakage must be provided (analogous to the leakage line in the idle chamber of the patent cited) since otherwise the system would block if too much leakage collects behind the spring chamber piston. If a diaphragm is used, a connection to the leakage between the idle volume and the spring reservoir is necessary, since because of the leakage at the pressure booster piston, the quantity of fuel in the idle chamber is not constant, and as the fuel quantity increases, the pressure rises and can block the pressure booster. This can be avoided by means of a suitable pressure limiting valve, which diverts the excess quantity.
In a fuel injection system, in particular a common rail system, having a high-pressure connection, which with at least one injector is in communication with a nozzle chamber, from which fuel is injected into the combustion chamber of an internal combustion engine, the above-stated object is attained in that at an arbitrary point between the high-pressure connection and the nozzle chamber of the injector, there is a pressure booster of the kind described above. The high-pressure connection line can communicate with a high-pressure reservoir, for instance, or can communicate directly with a high-pressure pump. During the injection, the idle volume of the pressure booster is displaced into the spring reservoir.
According to the invention, the displacement work required to displace the fuel outward and aspirate it can be reduced, if the flow losses from friction during aspiration/expulsion are greater compared to leakage. Thus the efficiency can also be improved.
A particular embodiment of the fuel injection system of the invention is characterized in that the pressure booster is integrated with the injector. In a common rail system, the pressure booster can also be integrated with the rail.
In general, such an arrangement can also be used in other injection systems that have a pressure booster of this kind. For instance, in a distributor pump, the 3/2-way valve upstream of the pressure booster can be omitted and replaced by a distributor with a corresponding 2/2-way valve.